Behavior of Concrete Bridge Decks Reinforced with High-Performance Steel

ACI Structural Journal, Jan/Feb 2008 by Seliem, Hatem M, Lucier, Gregory, Rizkalla, Sami H, Zia, Paul

Load-deflection behavior-The three bridge decks were subjected to loading and unloading to load levels of 50, 100, and 150 kips (222, 445, and 667 kN) per span, and then to failure. The load-deflection envelopes up to failure for the three tested bridge decks are shown in Fig. 5. It should be noted that the deflections shown in Fig. 5 are measured at the center of the respective deck span directly under the applied load. It can be seen from Fig. 5 that the first bridge deck reinforced with HP steel using the same reinforcement ratio as used for the actual bridge exhibited smaller deflections than that of the other two bridge decks at the same load level. The slightly higher stiffness of the first deck is likely due to the higher concrete compressive strength and to the higher strength of HP steel. Despite the lower reinforcement ratio used for the third bridge deck (33% less than that of the first two decks), it was capable of achieving the same ultimate load-carrying capacity as the second bridge deck reinforced with the Grade 60 steel. This behavior is attributed to the higher tensile strength of HP steel. The slight increase of the deflection measured for the third bridge deck compared with the second deck is due to the use of less steel and to the slight reduction of the modulus of elasticity of HP steel at high stress levels.

The deflection profiles along the transverse direction for the right span of the second and third bridge decks are shown in Fig. 6. It should be noted that the deflection profiles are plotted for the last loading cycle only. Therefore, residual deflections are shown at the beginning of the loading cycle (zero load). The deflection profiles indicate that the maximum deflection occurred at midspan under the applied load. The deflection profiles also show that the deflection behavior of the deck reinforced with reduced amount of HP steel is very similar to that of the deck reinforced with conventional Grade 60 steel.

Deflection profiles in the longitudinal direction for the right span of the second and third bridge decks are given in Fig. 7. It should be noted that the deflection profiles are plotted for the final loading cycle only. Accordingly, the deflections shown in Fig. 7 include the residual deflections from previous loading cycles. The longitudinal deflection profiles demonstrate the curvature in the longitudinal direction, implying the two-way flexural behavior of typical bridge decks under concentrated loads. In addition, the deflection profiles illustrate that the deflection at the edge of the decks was very small. This indicates that selection of the length of the test models is adequate for carrying the total load and, therefore, representative to typical bridge decks.

Crack pattern-No cracks were observed up to a load level of 50 kips (222 kN) for any of the three bridge decks. The first visible top cracks occurred at a load level of approximately 60 kips (267 kN) for each deck. According to the AASHTO Specifications,5 an axle of the design truck consists of a pair of 16 kip (71 kN) wheel loads spaced 6 ft (183 mm) apart. Therefore, at a load level of 21 kips (93 kN), which includes the dynamic allowance, the three bridge decks remained uncracked and the deflection at the service load level was identical for the three bridge decks. Therefore, reducing the amount of HP steel used in the third bridge deck did not alter the serviceability behavior.